Currently, the standardization of the standards of radio LAN (Local Area Network) is being prepared by IEEE (Institute of Electrical and Electronics Engineering). The standards of the radio LAN is defined as IEEE Std 802.11 (hereinafter referred as “IEEE 802.11”). At present, extension standards such as IEEE 802.11e are under consideration in order to carry out the extension relating to QoS (Quality of Service).
In IEEE 802.11, a method based on CSMA/CA (Career Sense Multiple Access with Collision Avoidance) is used as a method for media access control to avoid competition of radio (wireless) resources between two or more terminals. This CSMA/CA system is a communication system, which makes it possible to avoid collision of transmission data such as back-off by detecting carrier wave (carrier sense) transmitted from other terminal in a radio section when terminals transmit the data in addition to the detection as to whether there is idle space in the channel or not. As CSMA/CA system, communication systems called DCF (Distributed Coordination Function) and PCF (Point Coordination Function) are defined. General outline of DCF and PCF are as described below:
In DCF, idle or free time in the radio section is measured (carrier sense). When the radio section is DIFS (DCF InterFrame Space), which has no signal in a predetermined period, it is judged that signal can be transmitted. On the other hand, in case signal is present in the radio section during DIFS period, signal is transmitted after a certain period of time (idle time+back-off time) in DIFS period. In case there is no idle time in the radio section after waiting for the transmission, transmission of signal is kept under standby condition for the period of idle time.
On the other hand, PCF provides the function to operate while avoiding collision of signals with the other terminals, which occur during DCF. In case there is no signal for a certain period of time called PIFS (PCF InterFrame Space) in the radio section, PCF can be started. In this case, DIFS>PIFS, and the priority is given to the operation of PCF over the operation of DCF. Also, during PCF operation, the collision of signals between the terminals can be avoided by the communication utilizing polling system.
Regarding the communication of an acknowledgment signal Ack (receiving acknowledgment information) in data receiving or the communication during PCF, a time period called SIFS (Short InterFrame Space) is set, and the priority is given to the data communication with higher communication priority. It is designed in such manner that signals can be transmitted in case there is no signal only in SIFS period on the radio section. That is, in IFS, which acts as the standard for timing to start the transmission of signal, there are three types of IFS (i.e. DIFS, PIFS, and SIFS). Based on the relation of DIFS>SIFS, data communication with higher communication priority can transmit the signals by utilizing IFS of shorter period. As described above, according to IEEE 802.11, the time called IFS is used for media access control. Based on this IFS, it is tried to avoid collision of signals between the terminals.
In case the data is transmitted, a header is added to the data to be transmitted. This header contains essential information for transmitting and receiving of the data between the terminals, such as the information relating to parameters necessary for communication or to extension function. In some cases, header is added to control information (essential information) relating to a protocol such as control and is transmitted. Among the essential information, the information to be known by a plurality of terminals is transmitted, to make sure, by the most redundant method (standard or essential method receivable at all of the terminals).
However, in the communication based on TDMA (Time Division Multiple Access) system according to IEEE 802.11, the data communication on radio section is controlled (e.g. avoidance of collision) by performing temporal control (control of time base), e.g. by providing idle time such as IFS as described above.
When referring to a radio section according to time flow, the elapsed time in the radio section can be roughly divided to transmission time of the data to be transmitted and the other time. In the other time, idle time such as IFS and transmission time of header to be added to the data are included. In the present specification, idle time such as IFS or transmission time of header is called overhead or time required for overhead.
FIG. 9 is a schematical drawing to explain the change of ratio of the time required for overhead to transmission time of data in case transmission rate is turned to higher speed in the prior art. FIG. 9 shows the case where, at the transmission rate of 10 Mbps, the ratio of the time required for overhead to the transmission time of data is 1:5. In this case, if the transmission rate is increased from 10 Mbps to 100 Mbps, data transmission time is turned to shorter (quicker), but the time required for overhead is not changed (Transmission time of header is shortened, but here, description is given by neglecting the reduction of transmission time of header). As a result, the ratio of the time required for overhead to the transmission time of data is turned to 2:1. This means that data transmission efficiency to overhead is decreased.
As described above, under the condition where media access control is performed on time basis, if high speed transmission rate is needed and data transmission rate in physical layer is increased, elapsed time other than data (time required for overhead) is relatively increased, and this leads to the decrease of substantial throughput in the data transmission. Specifically, despite of the fact that data transmission rate has been turned to higher speed, the time required for overhead (in particular, IFS) remains the same. This means that the ratio of the data transmission time to the time required for overhead is increased, and this leads to aggravation of the data transmission efficiency. Also, in association with functional extension, the control information to be added is increased. When the capacity of the data transmitted as header is increased, the time for header transmission within overhead is increased.
FIG. 10 is a schematical drawing to explain the change of the ratio of the time required for overhead to the data transmission time in case data in the prior art is divided. When the data to be transmitted is divided to smaller packets for increasing the efficiency of re-transmission, occupying time (occupying ratio) of the overhead in the radio section is increased compared with the case the data is transmitted without dividing to packets. The capacity of the data transmissible within the same time period is decreased, and this means that the throughput of data transmission is decreased.